The decline of CD4+ T cells has long been recognized as one of the hallmarks of HIV-1 disease. However, the mechanism for this decline is still not known and is the subject of substantial controversy. One major limitation in studies of this decline has been the lack of a method fr measuring production of new lymphocytes and how this is affected by HIV-1 infection. The overall hypothesis is that the specific turnover rate of T lymphocyte populations will have an important impact on the rate of progression of HIV-1 disease in infected individuals. The specific aim of this study is to measure turnover (production and loss) of T helper and suppressor cell populations in the following individuals. Individuals who have had little or no progression of HIV-1 disease (as indicated by stable CD4+ cell counts) for 10 or more years, without effective drug therapy. Individuals who have had significant disease progression (as indicated by loss of CD4+ T cells at greater than the average rate) over 5 - 10 years of being infected with HIV-1, and who are not on effective drug therapy which could affect lymphocyte turnover rates. Individuals who are not infected with HIV-1 but are otherwise similar to the HIV-1 infected individuals who will be studied. Also to measure turnover of specific T helper and T suppressor subsets defined by monoclonal antibodies (e.g., naive cells, memory cells) in the above individuals.